I have seen Americans who didn't seem to be Hare Krishnas get Hindu priests to perform sacraments for the success of their business. Are Hindu priests allowed to do this according to scriptures?
The Brahmins who officiate as priests for the Sudras are condemned in the scriptures..
See the following verses from the Manu Smriti:
11.42. Those who, obtaining wealth from Sudras, (and using that) offer an Agnihotra, are priests officiating for Sudras, (and hence) censured among those who recite the Veda.
11.43. Treading with his foot on the heads of those fools who worship a fire (kindled at the expense) of a Sudra, the giver (of the wealth) shall always pass over his miseries (in the next world).
Similar things are said in other scriptures as well. Like the Vashishta Dharma Sutras 15.21 says:
A man who has divulged the Veda or officiated at a Sudra’s sacrifice, and those who have fallen from the rank of the highest class––for them the rite of overturning the water pot should be performed. slave, a son from a wife of a lower caste, or a relative of a lower class lacking good qualities should fetch a broken pot from a heap of unusable pots; spread on the ground some Darbha grass or Lohita grass with their tips cut off; and overturn the pot filled with water with his left foot for that person. 13 And his relatives, their hair dishevelled, should touch the man who overturned the pot. They may then return home as they wish, turning their left side towards that place
Now, assuming the American is not a Hindu living as that country's citizen, such an act will obviously not be allowed.. Because, the American won't be included in the 4-class Hindu Varna system.
So, when officiating for the Sudra is condemned what to say about officiating for one who does not even belong to the Varna-system?